1. Field of the Invention
This invention relates to an optical recording medium and its designing method.
2. Prior Art
Highlight is recently focused on optical recording media capable of recording information at a high density and erasing the recorded information for overwriting. One typical overwritable optical recording medium is phase change optical recording medium wherein the recording layer is irradiated with a laser beam to change its crystalline state whereupon a change in reflectivity caused by the change of the crystalline state is detected for reading of the information. The phase change optical recording media are of great interest since the medium can be overwritten by modulating the intensity of a single laser beam and the optical system of the drive unit is simple as compared to magnetooptical recording media.
When information is recorded on the optical recording medium of phase change type, the entire recording layer is typically first brought into crystalline state, and then, a laser beam of high power (recording power) is applied so that the recording layer is heated to a temperature higher than the melting point. In the region where the recording power is applied, the recording layer is melted and thereafter quenched to form an amorphous record mark. When the record mark is erased, a laser beam of relatively low power (erasing power) is applied so that the recording layer is heated to a temperature higher than the crystallization temperature and lower than the melting temperature. The record mark to which the laser beam of erasing power is applied is heated to a temperature higher than the crystallization temperature and then allowed to slowly cool to recover the crystalline state. Accordingly, in the optical recording media of phase change type, the medium can be overwritten by modulating the intensity of a single light beam.
The recording layer of phase change type is most often constituted from a material of Gexe2x80x94Sbxe2x80x94Te system or Agxe2x80x94Inxe2x80x94Sbxe2x80x94Te system which provides a substantial difference in reflectivity between the crystalline and the amorphous states as well as a relatively stable amorphous state.
In the phase change optical recording medium, grooves (guide grooves) are generally provided on the light-transmitting substrate for the purpose of tracking, and the grooves often carry address information. It has been commonplace in the past to form record marks either in the groove or in the area between adjacent grooves (land). However, land/groove recording wherein both the groove and the land are used for the recording track has been recently proposed in order to reduce the recording track pitch and enable high density recording. However, when the recording track pitch is reduced in the medium wherein the land/groove recording is conducted, beam spot of the laser beam used for the recording and reading will extend into the adjacent track, and this invites increased cross talk in the reading.
In view of such situation, Japanese Patent No. 2697555 proposes use of a medium wherein the land and the groove have a substantially same width and the groove has a depth in optical length of xcex/7 to xcex/5 to thereby reduce the cross talk in the land/groove recording provided that X is the wavelength of the reading beam. Japanese Patent No. 2697555 also discloses that the cross talk is minimized when the groove has a depth in optical length of xcex/6. Japanese Patent Application Laid-Open No. (JP-A) 7658/1999 discloses that the cross talk is minimized in the land/groove recording when the groove has a depth in optical length of xcex/6 to xcex/3. As a matter of fact, the groove depth of the commercialized DVD-RAM is 70 nm (optical length of approximately xcex/6).
Another effective countermeasure which may be taken for high density recording is use of a recording/reading laser beam with a smaller beam spot. The beam spot diameter is generally represented by xcex/NA when the laser beam has a wavelength xcex and the objective lens of the recording/reading optical system has a numerical aperture NA. Therefore, the beam spot diameter can be reduced either by reducing the wavelength of the recording/reading beam or by increasing the NA. For example, when DVD is compared to CD, the recording capacity of DVD (4.7 GB/side) is 6 to 8 times larger than that of the CD and such recording capacity is realized by reducing the wavelength of the recording/reading beam from 780 nm to 650 nm and increasing the NA from 0.45 to 0.6.
Use of such high NA, however, invites decrease in tilt margin. Tilt margin is tolerance for the tilting of the medium in relation to the optical system, and the tilt margin is determined by the NA. When the recording/reading beam enters the recording layer through a light-transmitting substrate and exits the medium again through the light-transmitting substrate and the light-transmitting substrate has a thickness t, the tilt margin is proportional to
xcex/(txc2x7NA3) 
It might be ideal that the surface of the medium is perpendicular to the direction of the recording/reading beam entrance. However, the surface is actually tilted from such ideal state. Lapse of time may also result in the deformation such as warping of the medium, and such deformation may result in the alteration of the tilt angle of the medium surface. Since increase in the jitter is invited when the medium is not accurately perpendicular to the recording/reading beam, widening of the tilt margin is quite important in practical point of view. It is also possible to provide a tilt sensor on the drive unit for feed back control. Such provision, however, is unpreferable in view of the increased cost.
In spite of the situation as described above, no proposal has so far been made for an optical recording medium of land/groove recording system wherein a sufficiently broad tilt margin has been realized at the same time with a sufficiently reduced cross talk. Investigation of the inventors of the present invention has also revealed that designing of a medium with the emphasis on the decrease of the cross talk may result in an insufficient output, and hence, in unduly increased jitter.
An object of the present invention is to provide an optical recording medium of land/groove recording system wherein a sufficient tilt margin and a reduced cross talk are realized simultaneously with a sufficiently increased output.
Such objects are attained by the present invention as described in (1) to (4), below.
(1) An optical recording medium having a light-transmitting substrate formed with grooves wherein the grooves and the region between adjacent grooves are used for the recording track, and a reading beam is irradiated through the light-transmitting substrate, wherein
the relations:
xe2x88x9212.6T+15.6xe2x89xa6xxe2x89xa625Txe2x88x923.8, and 
0.55xe2x89xa6Txe2x89xa60.68 
are satisfied when the reading beam has a wavelength of xcex, the light-transmitting substrate has a refractive index at the wavelength of n, the objective lens which is used in the reading optical system has a numerical aperture of NA, recording track pitch is PT, the groove has a depth of xcex/(xxc2x7n), and T is PT/(xcex/NA).
(2) An optical recording medium according to the above (1) wherein said medium is used in combination with a reading optical system wherein
NAxe2x89xa70.6. 
(3) A method for designing an optical recording medium having a light-transmitting substrate formed with grooves wherein the grooves and the region between adjacent grooves are used for the recording track, and a reading beam is irradiated through the light-transmitting substrate, wherein
the groove is designed to have a depth satisfying the relations:
xe2x88x9212.6T+15.6xe2x89xa6xxe2x89xa625Txe2x88x923.8, and 
0.55xe2x89xa6Txe2x89xa60.68 
when the reading beam has a wavelength of xcex, the light-transmitting substrate has a refractive index at the wavelength xcex of n, the objective lens which is used in the reading optical system has a numerical aperture of NA, recording track pitch is PT, the groove has a depth of xcex/(xxc2x7n), and T is PT/(xcex/NA).
(4) A method for designing an optical recording medium according to the above (3) wherein the medium is designed by assuming that:
NAxe2x89xa70.6. 